The absence of a return
mission reduces the radiation exposure from galactic cosmic rays. These cosmic
rays are hard to shield against without the use of a prohibitive shielding
mass, which would require more than 10 times the standard spacecraft shielding.
Reducing the time spent traveling through space — and, thus, the exposure from
the cosmic rays — is significant, as these rays are the source of 95 percent of
the radiation exposure, according to a recent paper published May 31 in the
journal Science.

For the study, Cary
Zeitlin of the Southwest Research Institute and his colleagues calculated
radiation exposure for a 360-day return trip to Mars to be 662 plus or minus
108 millisieverts(mSv), equivalent to the radiation of about 10,000 chest
X-rays. The researchers determined that value using measurements from NASA's
Mars Science LaboratoryRadiation Assessment Detector (RAD), onboardthespacecraft while the Curiosity rover traveled to Mars in 2012. [Photos: Mars
One Wants You as a Martian Colonist]

This finding from RAD
indicates that for a human traveling to and living on the Red Planet, the risk
of a fatal cancer resulting from radiation exposure lies within the currentlyaccepted risk for low-Earth-orbit astronauts.

Radiation exposure
during space travel

In transit to Mars, the
Mars One crew will be protected from the other source of radiation — solar
particles — by the structure of the spacecraft, which will provide 10 to 15
grams per square centimeter (2.3 to 3.4 ounces per square inch) of shielding, equivalent
to 4 to 6 inches (10 to 15 cm) of water.

That level of
protection will not suffice in the case of coronal mass ejections, solar flares
and other sources of solar particle events.

Taking their cue from
anonboard solar-flare forecasting and radiation measurement system the crew
will retreat to a dedicated radiation shelter located in a hollow water tank,
where shielding will increase to a total of 40 grams per square cm (9 ounces
per square inch).

Assuming the exposure
levels the RAD experienced, Mars One's crew will be exposed to 386 plus or
minus 63 mSv of radiation on its 210-day journey to Mars in 2022, which is
equivalent to 58 percent of the total exposure of a 360-day return trip, as
calculated by the recent Science study. [Could Radiation Harm Astronauts
Traveling to Mars? (Video)]

Radiation exposure on
Mars

On Mars, radiation
exposure is considerably less than the exposure in space. Thanks to the
shielding provided by the planet itself and its atmosphere, Mars' surface
receives 30 microsieverts (0.03 millisieverts) of radiation per hour during
solar minimum, according a NASA technical paper entitled "Space Radiation
Cancer Risk Projections for Exploration Missions: Uncertainty Reduction and
Mitigation."

The research, conducted
by Francis Cucinotta, chief scientist at the NASA Space Radiation Program and
his colleagues, found that the exposure is reduced by about 50 percent during
solar maximum because solar particles actually decrease galactic rays.

The Mars One habitat
will be covered by a necessary layer of soil that provides shielding even
against galactic cosmic rays. Sixteen feet (5 meters) of Martian soil providesthe same protection as the Earth's atmosphere — equivalent to 1,000 grams per
square cm(227.6 ounces per square inch) of shielding. The Mars One habitat can
support a soil layer 36 feet (11 m) thick. If the settlers spend, on average,
two hours per day outside the habitat, their individual exposure adds up to 22
mSv per year.

Radiation exposure
limits

The world's space
agencies have put in place radiation exposure limits for astronauts over theircareers. The agencies restrict the astronauts to serve low-Earth-orbit missions
and do not consider the longer durations and different conditions of
interplanetary missions.

NASA low-Earth-orbit
radiation exposure limits were designed to keep the risk of radiation-induced
fatal cancer below 3 percent, according to the National Council on Radiation
Protection and Measurements report 132. The limits depend on the sex and age of
the astronaut.

According to that
report, young females are most susceptible to radiation-induced fatal cancer
and therefore have the lowest allowed exposure. For International Space Station
(ISS) operations, NASA uses the 3 percent risk with a confidence level applied
to it, resulting in doses about three times lower than the acceptable limits,
according to National Council on Radiation Protection and Measurements report
142.

Considering a scenario
where an inhabitant receives the upper limit of radiation of 449 mSv during the
210 day transit to Mars, for a female inhabitant younger than 35, the exposure
amounts to about 3 percent risk for her lifetime (not including NASA ISS
confidence levels). Risks are lower for women departing at a higher age, and
especially for male colonists. On Mars, the risk can be managed by monitoring
each crew member's radiation exposure and limiting the surface exploration time
of those who are most at risk.

Comparing the risks

The risk from radiation
is visualized more easily when compared to other risks to which humans are
exposed.

For example, an average
person in the United States has a 21 percent risk of dying from cancer,
according to the American Cancer Society's Cancer Facts and Figures 2013. The
increased risk attributed to space travel is, on average, smaller than the risk
add-on from smoking: Lifelong smokers who smoke less than 25 cigarettes per day
raise their risk of dying of lung cancer from below 1 percent (for nonsmokers)
to 15.9 percent (men) or 9 percent (women), according to research conducted by
Richard Peto and colleagues at Radcliffe Infirmaryin the United Kingdom and
published in the British Medical Journal in 2000.

Even back on Earth, the
risk of dying from an adventure like climbing Mount Everest was 2 percent until
2006, according to AdventureStats.com.

Going to Mars, like
climbing the highest mountains, is not for everyone. And those explorers are
aware that mankind's great adventures will always be risky.

Mars One is evaluating
the risks involved in the mission, including those from radiation exposure, and
will clearly explain each risk to all stakeholders and, most importantly, toits crews.

One of Mars One's main
responsibilities is to find suitable men and women for the mission, and then
train and prepare them for the challenge. The crew members will ultimately
decide if the risk is worth the reward.

Colonizing Mars is the
next giant leap for humankind; radiation should not hold us back.

The views expressed are those of the author and do not necessarily
reflect the views of the publisher. This article was originally published onSPACE.com.

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